Portable terminals such as portable telephones and PDAs (Personal Digital Assistants) have recently spread remarkably. A factor in the rapid spread of these portable terminals is a liquid crystal display apparatus mounted as an output display unit of the portable terminals. The reason is that in principle, the liquid crystal display apparatus has a characteristic of not requiring much power for driving the liquid crystal display apparatus and is therefore a display device of low power consumption.
A portable terminal uses a single power supply voltage battery as a power supply. On the other hand, a logic unit and an analog unit of a horizontal driving circuit for writing information to each of plural pixels arranged in a matrix manner through a signal line in the liquid crystal display apparatus use different direct-current voltages. A vertical driving circuit for selecting the pixels by a unit of a row uses a direct-current voltage of a greater absolute value than the horizontal driving circuit. Accordingly, the liquid crystal display apparatus mounted in the portable terminal uses a power supply voltage converting circuit, or a so-called DC-to-DC converter (hereinafter described as a DD converter) for converting a single direct-current voltage into a plurality of direct-current voltages of different voltage values.
A conventional DD converter in a liquid crystal display apparatus generally uses an inductor L. However, with a recent reduction in power consumption and size of portable terminals, DD converters of a charge pump type have often been used. Although a charge pump type DD converter has a relatively low current capacity, the charge pump type DD converter does not require use of the inductor L as an external component. The charge pump type DD converter therefore has an advantage of being able to contribute to reducing the size of the portable terminal.
FIG. 1 shows a configuration of a charge pump type DD converter of a negative voltage generating type according to a first conventional example.
In FIG. 1, a Pch MOS transistor Qp101 and an Nch MOS transistor Qn101 are connected in series with each other between a power supply for supplying a single direct-current voltage VCC and a ground (GND). The Pch MOS transistor Qp101 and the Nch MOS transistor Qn101 have gates connected to a common point, thus forming a CMOS inverter 101. A pulse generating source 102 applies a switching pulse of a predetermined frequency to the gate common connection point of the CMOS inverter 101.
A drain common connection point (node A) of the CMOS inverter 101 is connected with one end of a capacitor C101. Another end of the capacitor C101 is connected with an anode of a diode D101 and a cathode of a diode D102. A cathode of the diode D101 is grounded. A load capacitor C102 is connected between an anode of the diode D102 and the ground.
In the thus formed DD converter of the negative voltage generating type, the power supply voltage VCC multiplied by −1, that is, a negative direct-current voltage −VCC is derived across the load capacitor C102 in principle.
FIG. 2 shows a configuration of a charge pump type DD converter of a voltage raising type according to the first conventional example. Fundamental configuration of the charge pump type DD converter of the voltage raising type is the same as that of the charge pump type DD converter of the negative voltage generating type. Specifically, in FIG. 2, the DD converter of the voltage raising type is different from the DD converter of the negative voltage generating type of FIG. 1 only in that a diode D101 is connected between another end of a capacitor C101 and a power supply (VCC). In the DD converter of the voltage raising type, twice the power supply voltage VCC, that is, a direct-current voltage 2×VCC is derived across a load capacitor C102 in principle.
However, since the thus formed charge pump type DD converters according to the first conventional example use clamping by the diode D101, an output voltage Vout does not reach the voltage value of the power supply voltage VCC multiplied by −1 or 2 even under no load, and is shifted by twice a threshold voltage Vth of the diode, as is clear from timing charts of FIG. 3 and FIG. 4. The timing charts of FIG. 3 and FIG. 4 show signal waveforms A to C at nodes A to C, respectively, in the circuits of FIG. 1 and FIG. 2, respectively.
The problem of the first conventional example has been improved by charge pump type DD converters according to a second conventional example as shown in FIG. 5 and FIG. 6. In FIG. 5 and FIG. 6, the same parts as in FIG. 1 and FIG. 2 are identified by the same reference numerals. FIG. 5 shows the DD converter of a negative voltage generating type, while FIG. 6 shows the DD converter of a voltage raising type. Fundamental configurations of the two DD converters are the same.
The DD converter of the negative voltage generating type will first be described. In FIG. 5, another end of a capacitor C101 is connected with a drain of an Nch MOS transistor Qn102 and a source of a Pch MOS transistor Qp102. A load capacitor C102 is connected between a source of the Nch MOS transistor Qn102 and a ground. A drain of the Pch MOS transistor Qp102 is grounded.
A gate common connection point of a CMOS inverter 101 is connected with one end of a capacitor C103. Another end of the capacitor C103 is connected with an anode of a diode D101 and gates of the Nch MOS transistor Qn102 and the Pch MOS transistor Qp102. A cathode of the diode D101 is grounded.
In the thus formed DD converter of the negative voltage generating type, an output voltage Vout reaches the voltage value of a power supply voltage VCC multiplied by −1 at no load, as is clear from a timing chart of FIG. 7.
On the other hand, the DD converter of the voltage raising type shown in FIG. 6 is different from the DD converter of the negative voltage generating type shown in FIG. 5 only in that switching transistors Qp103 and Qn103 are of an opposite conduction type; and a diode D101 is connected between another end of a capacitor C101 and a power supply (VCC). In the DD converter of the voltage raising type, an output voltage Vout reaches the voltage value of twice a power supply voltage VCC at no load.
The timing chart of FIG. 7 and a timing chart of FIG. 8 show signal waveforms A to C at nodes A to C, respectively, in the circuits of FIG. 5 and FIG. 6, respectively.
However, the thus formed charge pump type DD converter according to the second conventional example clamps switching pulse voltage for the switching transistors (MOS transistors Qn102 and Qp102), that is, voltage level of a node D at a voltage value resulting from a shift by a threshold voltage Vth of the diode D101. Therefore, a sufficient driving voltage may not be provided for the switching transistors, particularly the Pch MOS transistor Qp102.
Thus, transistor size of the Pch MOS transistor Qp102 needs to be set large. The increase in the transistor size results in a problem such as an increase in circuit area or a decrease in current capacity. In addition, when pumping operation is stopped temporarily in a power saving mode or the like, the clamped level of the switching pulse voltage is varied with a change in a duty ratio of the switching pulse. This results in a problem such as a decrease in current capacity.
The above problem becomes serious both when a threshold value Vth of the transistor is great and when variation in the threshold value Vth is great. When a circuit is formed on a glass substrate by using thin film transistors (TFTs), for example, the problem is an important consideration. It is known that amorphous silicon and polysilicon used to form the thin film transistors have inferior crystallinity and inferior controllability of the conducting mechanism to single-crystal silicon, and thus the formed thin film transistors have great variations in characteristics.
It is accordingly an object of the present invention to provide a power supply voltage converting circuit that can obtain a high current capacity on a small-area circuit scale, a control method thereof, a display apparatus having the power supply voltage converting circuit as a power supply circuit, and a portable terminal having the display apparatus.